Substituent Effect on the Molecular Geometry

As it is well known for almost a hundred years and what has become a subject of countless reports, substituents may affect significantly chemical reactivity and physicochemical properties of the molecules in question [73,74]. Their effect on molecular geometry became better known since X-ray diffraction techniques of structirre determinations began to be a readily accessible source of molecular geometry and particularly the data bases have become well supplied with precise structural data. 

An obvious consequence of the Bent-Walsh rule is that bond lengths a and bond angle a should be correlated. Unortunately this has never been found due to too low precision in bond lengths determination [82], However, if instead of bonds lengths a the differences A = b - a are taken, then the mutual correlation appears [83] as shown in Fig. 23. 

The dependence A vs a works since in the subtraction part of the experimental error cancel out, and hence the dependence predicted by the Bent-Walsh rule may be observed. It is worth mentioning that the line described by A vs a relationship for p-homo-disubstituted benzene derivatives may be a good reference line for the situations in which there is no x-electron cooperative effect. For p-X-Y-disubstituted derivatives, if X and Y differ significantly by their electron accepting/donating abilities, the appropriate points in this graph are shifted down, as shown in Fig. 24. 

It is worth mentioning that a similar picture is obtained in the case of EDA-complexes of N,N,N ,N -tetramethylphenylenediamine, as shown in Fig. 25. 

the greater is the charge transfer, this time an intermolecular charge transfer from the phenylenediamine derivative onto the accepting molecule in the EDA complex or anion in the salt, the greater is the observed shift of A from the reference line obtained for the A vs a plot for para-homodisubstituted benzene derivatives. 

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